asthenosphere

Image credit: Massey University.

The asthenosphere is the uppermost layer of the mantle of a rocky planet, located below the lithosphere.
On Earth, this zone of soft, easily deformed rock lies at depths of about
100 to 250 kilometers. It is more plastic than adjacent layers because the combination
of pressure and temperature places it near (or slightly above) the melting
point. Asthenospheric movements may disrupt the planet's surface.